To establish the infiltration rate of the soil excavate a soakage trial pit to the same depth as anticipated in the full size soakaway. For run-off from 100 square metres this will be 1 to 1.5m below the invert level of the drain discharging to the soakaway.

The trial pit should be 0.3 to 1m wide and 1 to 3m long. It should have vertical sides trimmed square and, if necessary for stability, should be filled with granular material.

When granular material is used, a full-height, perforated, vertical observation tube should be positioned in the pit so that water levels can be monitored with a dip tape.

Calculate the soil infiltration rate from the time taken for the water level to fall from 75% to 25% effective storage depth in the pit, using the lowest f value of three test results for the design:

Soil infiltration rate, f = (Vp75 - 25) / (ap50 x tp75 - 25)


Vp75 - 25 = the effective storage volume of water in the trial pit between 75% and 25% effective depth

ap50 = the internal surface area of the trial pit up to 50% effective depth and including the base area

tp75 - 25 = the time for the water level to fall from 75% to 25% effective depth.

To further assist the designer a soil infiltration table has been provided.

This table should only be used to give an indication of typical soil infiltration rates.

A full site investigation must be carried out to assess the actual rate.